Tens of millions of tons of phosphorus-containing minerals, such as apatite, rock phosphate, etc., are processed for producing phosphorus-containing fertilizers. Typically, the processing is carried out by treating these natural materials with concentrated nitric or sulphuric acid. During the treatment with sulphuric acid, apatite is decomposed with precipitation of calcium sulphate and formation of phosphoric acid solution. In the course of the hemihydrate process, 65%-85% of REM are precipitated with phosphogypsum, and the rest of REM are dissolved in the wet process phosphoric acid. For example, apatite of the Kola Peninsula contains up to 1% of rare-earth metals, 70 to 80% of which are precipitated with calcium sulphate in the course of the hemihydrate processing at apatite with sulphuric acid. All REM which remain in phosphoric acid further goes to fertilizers produced therefrom. Thus, to perform the complete extraction of REM from apatite, it is necessary to recover REM from both phosphogypsum and the acid. It is associated with the complexity of production and increasing the required capital investment. Therefore, it is desirable to concentrate REM in one of these products, either wet process phosphoric acid or phosphogypsum.
A method comprising treatment for 20-25 minutes with 22-30% sulfuric acid solution is carried out to provide extraction of rare earth elements and compounds of phosphorus in solution followed by separation of insoluble residue in the form of gypsum which contains a considerable amount of sulfuric and phosphoric acid is described in RU patent No. 2337897. The extraction solution also contains double sulfates of rare earth metals with sodium or potassium. The resulting crystalline gypsum is treated with Ca(OH)2 or CaO or CaCO3 to neutralize residual sulfuric and phosphoric acid to reach pH>5. The content of phosphorus impurities in the mother liquor shall be monitored. And depending on the ratio of its content and residual humidity gypsum, the mother liquor is directed to the extraction step or subjected to purification treatment by adding TiOSO4*H2O to reach an acceptable level of P2O5. This method allows the recovery of up to 82.1% REM in the extraction solution with subsequent isolation thereof by crystallization in the form of concentrate with a degree of recovery up to 68.5%. The disadvantages of this method include obtaining crystalline gypsum with a pH which does not meet the standard requirements for construction and has a high content of P2O5. Isolation of the lanthanides from the oversaturated extraction solution requires a significant period of time (2 hours). Monitoring of content of phosphorus impurities in the acid extractant and residual moisture of gypsum precipitate is required. To remove the phosphorus excess, it is necessary to have the equipment to neutralize phosphorus compounds by titanium compounds in a dry form or in a mixture with concentrated sulfuric acid followed by separation of titanyl phosphate and obligatory treatment with concentrated sulphuric acid. If the concentration of sulphuric acid in the extraction solution is reduced to below 22%, an increase of its concentration is necessary in order to reuse it in the process. It requires a large number of reactors, capacitive, filtration and other equipment for extraction of rare earth elements, storage of various extraction solutions and neutralization of phosphorus compounds.
A method of recovering rare-earth elements from phosphogypsum is disclosed in RU patent No. 2293781. The method comprises treatment of phosphogypsum with sulphuric acid solution to recover rare-earth elements into solution, separation of gypsum precipitate, increasing the oversaturation rate of the solution in terms of rare-earth elements to crystalize rare-earth metal concentrate, and separation of the concentrate from mother liquor followed by concentrate processing. Phosphogypsum is treated with 22-30% sulphuric acid solution at liquids-to-solids ratio 1.8-2.2 during 20-30 min to prevent spontaneous crystallization of rare-earth element concentrate in solution before insoluble precipitate is separated. An increase of the oversaturation rate of the solution is achieved by providing sodium concentration 0.4-1.2 g/L. The disadvantage of this method is the use of additional reagents, high acid concentrations and significant amounts thereof, a large number of basic technological operations with incomplete extraction of rare earth elements (up to 71.4%) and the overall complexity of the process.
Concentrating REM from solutions in solid phase is required not only after its recovery from phosphogypsum into the target solution, but also, for example, during REM extraction from wet process phosphoric acid in the course of apatite processing with sulphuric acid. A method for extracting REM from the wet process phosphoric acid and concentrating REM on the solid sorbent using the acid retention method applying the effect of isothermal saturation in adsorbing layers is cited here (Khamizov R. H., Krachak A. N., Gruzdeva A. N., Bolotokov A. A., Khamizov S. H., Smirnov A. A., Zhiguleva T. I. “Sorption concentration and isolation of REM from wet process phosphoric acid”. Sorption and chromatography processes (rus), 2012, V. 12, No. 1, pp. 29-39). One of the simplest variants of the acid retention method was used in this work. Repetitive operating cycles are carried out: processed solution is passed upwards through ion-exchange column filled with a dense layer of strong base anion resin, previously equilibrated with the processed solution of the phosphoric acid and then washed out with water, until the acid concentration at the outlet is equal to the concentration of the initial solution and then inverse elution of the acid was carried out with water being passed down. The main effect is that the first passing fractions contain practically no or very little acid (up to pH 2-3). REM salt and some other components poorly soluble in such conditions form a precipitate at the column outlet. Salts soluble in such conditions, for example calcium and manganese compounds, are even concentrated and can be detected in outgoing fractions of solution, corresponding to phosphorus output. Part of REM remained in the solution is also detected in these fractions.
The main disadvantage of this method is its relatively low productivity limited by capacity of the ion-exchange resins and, therefore, high price thereof.